1. General Structure of Driving Stability Control (DSC)
The term driving stability control (DSC) covers four principles of influencing the driving behavior of a vehicle to by means of predeterminable pressures in individual wheel brakes and by interfering with the engine management of the driving engine. These include the anti-locking system (ABS), which is to prevent the locking of individual wheels during a braking process; the traction slip control system (TSC), which is to prevent the spinning of the driven wheels; the electronic brake effort proportioning system (EBV), which controls the ratio of the brake efforts between the front axle and the rear axle; and a yawing moment control system (YMC), which ensures stable driving conditions during travel in a curve.
Consequently, a vehicle is defined in this connection as a motor vehicle with four wheels, which is equipped with a hydraulic brake system. In a hydraulic brake system, a brake pressure can be built up by the driver by means of a pedal-actuated main cylinder. Each wheel has a brake, with which one inlet valve and one outlet valve each is associated. The wheel brakes communicate with the main cylinder via the inlet valves, while the outlet valves lead to a pressureless tank or to a low-pressure accumulator. Finally, there also is an auxiliary pressure source, which is able to build up a pressure in the wheel brakes regardless of the position of the brake pedal. The inlet and outlet valves can be electromagnetically actuated for pressure regulation in the wheel brakes.
To detect states in the dynamics of the vehicle movement, there are four speed sensors, one per wheel, one yaw rate meter, one lateral acceleration meter, and at least one pressure sensor for the brake pressure generated by the brake pedal. The pressure sensor may be replaced with a pedal travel or pedal force meter if the auxiliary pressure source is arranged such that a brake pressure built up by the driver is not distinguishable from that of the auxiliary pressure source.
A fall-back solution is advantageously put into practice in light of such a large number of sensors. This means that, in the case of failure of part of the sensor system, only the component of the control system that depends on that part is switched off. If, for example, the yaw rate meter fails, no yawing moment control can be performed, but the ABS, TSC and EBV continue to function. The driving stability control can consequently be limited to these other three functions.
In a driving stability control, the driving behavior of a vehicle is influenced such that the driver will be better able to control the vehicle in critical situations, or critical situations will be avoided to begin with. A critical situation is defined herein as an unstable driving condition in which, in the extreme case, the vehicle does not follow the driver's instructions. The function of the driving stability control is consequently to impart to the vehicle the behavior desired by the driver in such situations within the physical limits.
While the longitudinal slip of the tires on the road surface is mainly of significance for the anti-locking system, the traction slip control system and the electronic brake effort proportioning system, the yawing moment control system (YMC) also involves additional variables, e.g., the yaw rate .PSI..
Various vehicle reference models may be used for yawing moment control. The calculation is simplest on the basis of a single-track model, i.e., the front wheels and the rear wheels are integrated in this model into one wheel each, which is located on the longitudinal axis of the vehicle. The calculations become considerably more complicated if they are based on a two-track model. However, since lateral displacements of the center of gravity (rolling movements) can also be taken into account in the two-track model, the results are more accurate.
The system equations ##EQU1## can be written in the phase space diagram.
The side slip angle .beta. and the yaw rate .PSI. represent the phase variables of the system. The input variable acting on the vehicle is the steering angle .delta., as a result of which the vehicle receives the yaw rate .PSI. as an output variable. The model coefficients c.sub.ii are formed as follows: ##EQU2##
c.sub.h and c.sub.v are the resulting rigidities from the tire, wheel suspension and steering elasticity on the rear axle and the front axle, respectively. l.sub.h and l.sub.v are the distances of the rear axle and the front axle, respectively, from the center of gravity of the vehicle. .THETA. is the moment of inertia about the yaw axis of the vehicle, i.e., the moment of inertia of the vehicle around its vertical axis.
Longitudinal forces and displacements of the center of gravity are taken into account in this model. This approximation is also valid only for low angular velocities. Consequently, the accuracy of this model decreases with decreasing curve radii and increasing velocities. However, the amount of calculations is manageable. Further explanations of this single-track model can be found in the book Fahrwerktechnik: Fahrverhalten Chassis Engineering: Driving Behavior! by Adam Zomotor, Vogel Buchverlag, Wurzburg, 1987.
A two-track model, whose accuracy is superior to that of a single-track model, is proposed for a vehicle in DE-40 30 704 A1. The yaw rate .PSI. and the side slip angle .beta. form the phase variables in this case as well. However, when a two-track model it used, it is necessary to consider the fact that an enormous calculation capacity is needed to make it possible to perform a control intervention in a relatively short time.
The problem that the necessary control intervention must be recognized in real time also arises in regard to the different functions of a driving stability control. It is of decisive significance that the suitable control intervention be rapidly recognized and performed with suitable means. In a vehicle which has an antilock brake system (ABS), a traction slip control (TSC) system, a system (EBV) for distributing brake effort between the front and rear axles and a yawing moment control (YMC) system, the task that arises is to structure the system such that the suitable control intervention will be performed in a sufficiently short time, before a critical driving situation leads to an accident.